In general, an organic electroluminescence (hereinafter referred to as an EL) display is one of flat plate type displays. The organic EL display includes an anode layer and a cathode layer formed on a transparent substrate, and an organic light-emitting layer is interposed between the anode layer and the cathode layer. The organic EL display has very thin thickness and it is fabricated as a matrix pattern.
Such an organic EL display is driven at a low voltage not greater than 15 Volts, and it exhibits advanced characteristics in terms of brightness, viewing angle, response time, power consumption, and so forth, compared to other types of displays, for example, a TFT-LCD. Besides, the organic EL display has a response time of about 1 μs, which is much faster than other displays, and, therefore, it is suitable for use in a next-generation multimedia display to which a function of implementing motion pictures is essential.
Fabrication of the organic EL display includes in general the steps of coating an insulating layer and a separator, both of which are made of an electrically insulating material, in order on a substrate on which an anode layer is formed and patterning an organic light-emitting layer and a cathode layer through an overhang structure of the separator.
Here, the insulating layer is formed on the entire surface of the anode layer except on dot-shaped openings defining pixels, and the insulating layer serves to prevent a leakage of a current at an edge portion of the anode layer.
Moreover, the separator formed on the insulating layer is arranged in a predetermined interval such that it crosses the anode layer. Further, the separator is configured to have an overhang structure with a negative profile, and it functions to separate the cathode layer between neighboring pixels.
Accordingly, both the insulating layer and the separator are necessary for a stable fabrication of the organic EL display.
For the reason, there have been proposed various methods for manufacturing an organic EL display by forming an insulating layer and a separator through a simplified process.
First of all, disclosed in U.S. Pat. No. 5,701,055 (hereinafter referred to Reference 1) is a manufacturing method for an organic EL display, in which an exposure and a developing process are conducted for each of two layers of photoresist layer, to thereby form an insulating layer and a separator individually.
In the method disclosed in Reference 1, an anode layer made of, e.g., an indium tin oxide (ITO), is formed on a transparent substrate in the shape of parallel stripes. Then, an insulating layer formed of, e.g., a positive photoresist layer is coated on the substrate on which the anode layer is provided.
Thereafter, the insulating layer is patterned through a photolithography process including an exposure process and a developing process such that it remains only on areas between the anode stripes and also on areas crossing the anode stripes. As a result, the insulating layer is patterned such that it exists on the entire surface of the anode layer except on dot-shaped openings patterned on the anode layer. That is to say, the insulating layer is patterned to have a lattice structure. Here, the openings define pixels of the organic EL display.
Afterward, a negative photoresist layer or the like is coated on the insulator pattern, and a separator with a negative profile is obtained by patterning the negative photoresist layer through a photolithography process including an exposure and a developing processes. At this time, the separator is arranged on the insulator pattern formed between the dot-shaped openings to cross the anode stripes, are configured to maintain a predetermined internal therebetween. Further, the separators have an overhang structure with a negative profile to allow a cathode layer, which is to be formed later, to prevent from occurring a short-circuit due to the connection to neighboring pixels. That is to say, the separator is formed to maintain a negative profile by using a characteristic of the negative photoresist layer. Therefore, a short circuit between cathode layers of neighboring pixels can be prevented.
Afterward, an organic light-emitting layer and a cathode layer are sequentially deposited on the resultant structure having the separators by using a metal mask. In this connection, when the organic light-emitting layer is deposited on the anode layer in the openings, there is a likelihood that the thickness of the organic light-emitting layer is reduced near the separator because of a shadow effect due to the separator, thus causing a short circuit between the cathode layer deposited on top of the organic light-emitting layer and the underneath anode layer. However, this problem is prevented by the presence of the insulating layer with a positive profile that is formed below the separator.
In accordance with the method disclosed in Reference 1 described so far, a reliable organic EL display can be fabricated by defining pixels and patterning an organic light-emitting layer and a cathode layer by using an insulating layer and a separator that are formed individually. In the conventional method in Reference 1, however, the photolithography process needs to be performed two times to form the insulating layer and the separator individually. Therefore, the manufacturing process for the organic EL display becomes complicated and manufacturing costs increases. Furthermore, since the insulating layer and the separator are respectively formed on top of each other as separate layers, adhesiveness therebetween may be weak. Thus, in case a ratio of width to height of a device isolation structure including the insulating layer and the separator decreases, the device isolation structure may be broken.
Since the method described in Reference 1 has such problems as mentioned above, there has been a demand for a further advanced method for fabricating an organic EL display by forming both an insulating layer and a separator as a single layer through a simplified manufacturing process.
Korean Patent No. 408,091 (hereinafter referred to as Reference 2) discloses one of such methods.
The method described in Reference 2 involves forming an insulating layer and a negative-profile trench serving as a separator through patterning an image-reversal photoresist layer of a single layer by performing an exposure process two times, and also performing an exposure process one time and a developing process two times using a half tone mask. Detailed description of the method will be provided below.
As in the method described in Reference 1, an anode layer made of, e.g., an ITO is formed on a transparent substrate in the shape of a plurality of parallel stripes. Then, an image-reversal photoresist layer is coated on the transparent substrate on which the anode layer is provided. Thereafter, a first exposure process using a half tone mask and a developing process are performed, whereby the image-reversal photoresist layer is patterned such that it only remains areas between the anode stripes and areas crossing the anode stripes. Thus patterned photoresist layer becomes to exist on the entire surface of the anode layer except on dot-shaped openings. That is, the photoresist layer has a lattice structure, and the openings define pixels.
Meanwhile, in the patterning step using the half tone mask, the image-reversal photoresist layer between the anode stripes is firstly exposed through a half tone pattern of the half tone mask and becomes to have a thinner thickness than its other areas crossing the anode stripes.
Thereafter, the image-reversal photoresist layer crossing the anode stripes is secondarily exposed to light through an exposure mask that shields the trench regions which is to serve as a separator. Then, an image-reversal baking process and a third exposure process (a flood exposure process) are performed to change the property of the image-reversal photoresist layer. Due to the characteristic of the image-reversal photoresist layer, during the image-reversal baking process, the portions of the photoresist layer secondarily exposed to light are cross-linked and still remain after a second developing process without being affected by the entire surface exposure process. Further, the image-reversal photoresist layer present in the trench regions, which is not exposed to light during the second exposure process, maintains its inherent property of the positive photoresist layer, and thus is removable during the second developing process which will be performed after the entire surface exposure process.
If the second developing process is conducted afterward, a negative-profile trench with an overhang structure is formed on the area of the photoresist layer crossing the anode stripes, wherein the trench serves to as a separator.
In accordance with the manufacturing method as described above, an insulating layer for defining pixels can be formed by using the image-reversal photoresist layer and, at the same time, a trench serving as a separator can be formed on the portions of the insulting layer crossing the anode stripes.
The subsequent processes for forming an organic light-emitting layer and a cathode layer are identical to those described in Reference 1, and therefore, detailed description thereof will be omitted.
In accordance with the manufacturing method disclosed in Reference 2, both an insulating layer and a trench serving as a separator can be formed by using an image-reversal photoresist layer of a single layer and a half tone mask. Therefore, the manufacturing method in Reference 2 is simpler than the method of Reference 1, and, also, the problem of adhesion between the insulating layer and the separator can be improved.
The method in Reference 2, however, also has disadvantages in that manufacturing costs for the organic EL display increases greatly as a result of using the half tone mask of a high price. Furthermore, design of the half tone mask is very difficult, and the manufacturing process is still complicated because the exposure step and the developing step should be performed three times and two times, respectively, to form the insulating layer and the trenches serving as the separators.
Besides, theoretically, though a remainder of the image-reversal photoresist layer other than its portions where the trenches are to be formed, i.e., the insulating layer needs to be cross-linked completely by the image-reversal baking not to be affected by the entire surface exposure process and the second developing process, some of the edge portions of the insulating layer may not be cross-linked completely by the image-reversal baking and thus can be removed during the second developing process. In particular, as for the insulating layer in which trenches are formed, if a part of the edge portions of the insulating layer adjacent to pixels is removed by being affected by the entire surface exposure and the second developing process, the inclined angle of the positive profile of the insulating layer will increase close to a right angle. As a consequence, the thickness of an organic light-emitting layer near the edge portion of the insulating layer will be reduced during the subsequent processes forming the organic light-emitting layer and a cathode layer, resulting in a short circuit between the cathode layer and the underneath anode layer.
Therefore, there has been a demand for still another method for manufacturing an organic EL display, while solving the above-mentioned problems. The Application of PCT/KR2004/002366 (hereinafter referred to as Reference 3) filed by the inventors of the present invention provides a method capable of solving some of theses problems of the conventional methods. In the method disclosed in Reference 3, an insulating layer and a separator is formed by patterning an image-reversal photoresist layer of a single layer by way of performing an exposure process and a developing process three and two times, respectively, by means of using a general exposure mask. Detailed description of this method will be provided hereinafter.
As similar as in the methods in Reference 1 and Reference 2, an anode layer made of, e.g., an ITO is formed on a transparent substrate in the shape of a plurality of parallel stripes. Then, an image-reversal photoresist layer is coated on the transparent substrate on which the anode layer is patterned. Thereafter, a first exposure and developing process is conducted by using a general exposure mask, to thereby perform a patterning of the image-reversal photoresist layer such that the photoresist layer only remains between the anode layers and on certain areas crossing the anode layers.
Afterward, the image-reversal photoresist layer is subjected to a second exposure thorough the use of an exposure mask for defining a region on which a separator will be formed. Then, the image-reversal photoresist layer is undergone through an image-reversal baking process, through which the characteristic of the image-reversal photoresist layer is changed. Subsequently, a flood exposure process (a third exposure process) is conducted. Due to the characteristic of the image-reversal photoresist layer, during the image-reversal baking process, a portion of the photoresist layer secondarily exposed to light, where the separator will be formed, is cross-linked and is left even after a second developing process without being affected by the entire surface exposure. Further, the image-reversal photoresist layer unexposed to light during the second exposure process maintains the characteristic of the original positive photoresist layer, and thus is removable during the second developing process performed after the entire surface exposure process.
Further, during the entire surface exposure process, an exposure energy can be adjusted such that the portion of the image-reversal photoresist layer, which is not exposed to the second exposure, is not completely removed by the second developing process but remains with a thickness thinner than that of the separator, to thereby be allowed to serve as an insulating layer for defining pixels.
Then, if the second developing process is performed, the portion of the photoresist layer exposed to light in the second exposures process are left and thus a negative-profile separator with an overhang structure is obtained. Further, the photoresist layer's portions not exposed to light are also left with its thickness reduced thinner than that of the separator, thus serving as an insulating layer for defining pixels. Subsequent processes for forming an organic light-emitting layer and a cathode layer are identical to those described in Reference 1 or 2, so detailed description thereof will be omitted.
The above-described method disclosed in Reference 3 has a merit in that an insulating layer and a separator can be formed by using an image-reversal photoresist layer of a single layer without having to use a high-price half tone mask with a design feature difficult to be fabricated. Therefore, by employing the method in Reference 3, some of the problems of Reference 2 can be solved.
Since, however, the method in Reference 3 also requires performing the exposure process and the developing process multiple times (3 times of exposure process and 2 times of developing process), the whole process for manufacturing an organic EL display is still complicated.
Moreover, there still exists the problem of Reference 2 that the portions of the image-reversal photoresist layer not exposed to light during the second exposure process, i.e., some of the edge portions of the insulating layer may not be completely cross-linked by the image-reversal baking and thus can be removed through the second developing process. That is to say, even in accordance with the method of Reference 3, a part of the edge portion of the insulating layer adjacent to pixels may be removed by being affected by the entire surface exposure and the second developing process, resulting in an increase of the inclined angle of the positive profile of the insulating layer close to a right angle. As a consequence, the thickness of an organic light-emitting layer near the edge portion of the insulating layer will be reduced during the subsequent processes for forming the organic light-emitting layer and the cathode layer, resulting in a short circuit between the cathode layer and the underneath anode layer.